1. Field of the Invention
The present invention relates to printers, and more particularly to dot matrix line printers in which individual hammers of a reciprocating hammer bank are selectively released to print characters and other information on a paper or other printable medium in dot matrix fashion.
2. History of the Prior Art
Line printers in which a hammer bank is reciprocated relative to a paper or other printable medium with hammers mounted along the length of the hammer bank being selectively actuated to impact the paper and thereby effect printing in dot matrix fashion are well known as shown, for example, by U.S. Pat. No. 3,941,051 of Barrus et al. In the Barrus et al patent which is commonly assigned with the present application, a plurality of elongated, thin, flat hammer springs are mounted in generally parallel, spaced-apart relation at a first end of each spring along the length of an elongated hammer bank structure. Dot impacting elements are mounted adjacent the opposite second ends of the hammer springs. Magnetic circuits between the opposite first and second ends of each hammer spring are completed by a permanent magnet and associated magnetic return path member of elongated configuration which are common to all of the hammers. The magnetic return path member mounts a plurality of pole pieces along a portion thereof opposite the permanent magnet. Each pole piece extends into contact with the upper second end of a different one of the hammer springs and is provided with a hammer release coil.
Each hammer spring is normally held in a retracted position by action of the permanent magnet. The force of the permanent magnet may be temporarily overcome so as to release the hammer spring by momentary energization of the coil mounted on the associated pole piece. This allows the hammer spring to fly into a neutral position where the hammer has attained maximum kinetic energy. At this point the dot imprinting tip mounted thereon impacts the paper and an included ink ribbon to print a dot. Upon impacting of the paper and ribbon, the hammer spring rebounds from the paper and ribbon and moves back to the retracted position in which it resides against the associated pole piece under the influence of the permanent magnet.
Arrangements of the type shown in the previously referred Barrus et al patent are capable of precise, on-the-fly printing so as to be capable of printing at speeds well in excess of 300 lines per minute where each line can require as many as 10 separate sweeps of the hammer bank and can contain as many as 132 characters, each of which is 5 dot columns wide. To print in this fashion release of the hammer springs must be precisely controlled. A sufficient amount of current must be provided to each coil over a proper time interval so as to allow the hammer to release, impact the paper with a desired amount of force and then rebound into the retracted position in a rapid and efficient manner in preparation for the next dot position. Once these precise operating conditions are established they must be maintained to provide uniformity in printing.
One problem which results in nonuniform hammer release characteristics and therefore nonuniform printing relates to the changing magnetic characteristics of the hammer bank as different numbers of the hammers are released. The use of certain common components in the hammer bank including the permanent magnet and the magnetic return path member as well as the relatively close proximity of the hammer springs to one another results in changes in such things as the reluctance of the magnetic path as the number of hammers being released changes. The result is that when a relatively large number of hammers are released, each release tends to be relatively slow or less than fully complete, resulting in the hammer spring flying into the print position with less than the desired velocity and kinetic energy. The result is that impacting may not be hard enough to maintain the desired dot density. Also, the retraction of the hammer spring may be difficult to accomplish, particularly in the short amount of time required to ready the hammer bank for the next dot position across the print paper. The problem is aggravated with the design of new hammer banks in which the number of hammers may be increased and the hammers themselves may be placed closer together and designed to undergo a shorter stroke in order to accomplish even higher printing speeds. Such factors result in even greater nonuniformity of operation depending upon the number of hammers released at each dot position.
Nonuniform hammer release characteristics can result from other factors in addition to variations in the number of hammers released. One such common factor is variations in the power supply. Such variations can ultimately affect the amplitude of current applied to the hammer release coils and thereby the timing and force of hammer release as the hammer bank moves to successive dot positions.
Accordingly, it is an object of the present invention to provide an improved hammer bank arrangement in a dot matrix line printer.
It is a further object of the invention to provide a printer hammer bank arrangement which provides for generally uniform release characteristics despite variations in operating parameters of the hammer bank arrangement.
It is a further object of the invention to provide a printer hammer bank arrangement which substantially reduces or minimizes nonuniformities in the hammer release characteristics which can occur as different numbers of hammers are released.